This invention relates to an apparatus for reproducing a selected image by electrostatically attracting development fluid onto a receiving medium. More particularly, the invention relates to patterned means having a plurality of pockets adapted to carry metered amounts of development fluid for deposition onto the receiving medium, and doctoring means for removing excess development fluid from the patterned means prior to deposition.
Fluid development of electrostatic images is not new, an early system embodying such a concept being disclosed in U.S. Pat. No. 3,084,043 issued to R. W. Gundlach on Apr. 2, 1963 and assigned to the instant assignee. Though specific prior art fluid development systems vary, a typical apparatus may include a rotatable drum having a photoconducting surface and an electrically conductive substrate. Receiving means such as a transparent interposer or any other suitable means may be moved between a roller and the rotatable drum, thereby contacting a portion of the photoconducting surface at all times.
In such liquid development systems, a portion of the photoconducting surface, not then rotated into contact with the transparent interposer, is electrostatically charged by any suitable means. The charged portion of the photoconducting surface is then subjected to a light and shadow representation of the image to be developed. As a result, the charged portion of the photoconducting surface is discharged through the conductive substrate at points struck by the lighted portion of the light and shadow representation of the selected image. Accordingly, the original charge applied to the photoconducting surface, upon receipt of the light and shadow representation, assumes a pattern corresponding to the image to be developed.
Electrostatic imaging systems of the prior art further include a source of development fluid, and means such as an intricately patterned gravure roll, adapted to receive development fluid from said source. Upon receipt of development fluid, a doctoring blade is conventionally applied to the gravure in an effort to remove excess fluid which would otherwise interfere with the accurate reproduction of the selected image. The doctored, fluid-carrying gravure roll is then typically rotated into facing relationship with the rotatable drum, at a point where the transparent interposer is in contact with the photoconducting surface. Development fluid is then electrostatically attracted to the portion of the interposer overlying the charged portion of the photoconducting surface.
Upon receipt of the development fluid, the interposer is moved into contact with a copying medium such as a sheet of copy paper. A pressure roller, adapted to urge the copy paper against the interposer, facilitates passage of development fluid from the interposer to the paper. Since the development fluid on the interposer is typically in the form of the selected image, that image is transferred from the interposer to the paper. After development, the photoconducting surface is discharged through the conductive substrate, thereby eliminating any residual electrostatic charges remaining on the photoconducting surface. At substantially the same time, excess development fluid on the transparent interposer is removed by means such as a doctor blade, thereby readying the apparatus for the subsequent development of other selected images.
Though fluid development systems of the type described have been used to reproduce selected images, they are subject to numerous drawbacks and deficiencies. For example, it is difficult to transfer accurate quantities of development fluid to the intricately patterned gravure roll for subsequent deposition onto receiving means such as a transparent interposer. As a result, insufficient amounts of fluid are sometimes attracted to the interposer, thereby preventing portions of the selected image from being fully developed. Alternatively, too much fluid is often attracted to the interposer, causing an undesirable blotching effect. Accordingly, it is a primary object of this invention to provide means for assuring that the proper amount of development fluid is passed from the gravure roll.
Another problem frequently associated with fluid development systems of the type described relates to the doctoring of excess ink from the gravure. More particularly, the doctoring blade frequently used in conventional fluid development systems must be applied with sufficient force to insure removal of all excess fluid from the gravure roll. If such excess fluid is not removed, blotching will inevitably occur. However, the relatively large forces applied to the gravure by the doctor blade frequently damage the intricate pattern of the gravure. Fabricating the gravure roll from a harder substance than the doctor blade will not necessarily solve the problem since the hardened gravure may then damage the blade during doctoring, thereby preventing proper removal of excess fluid from the gravure roll. It is therefore another object of the invention to provide doctoring means for removing sufficient fluid from the gravure without damaging the intricate pattern thereon; and to provide a gravure roll which will not damage the blade during doctoring.
The necessarily large forces applied to the gravure roll by conventional doctoring means cause additional problems in fluid development systems of the type described. For example, when the doctor blade is forcefully applied to the gravure, it is often urged into the pockets, thereby withdrawing fluid therefrom. As a result, there may be too little fluid remaining in the pockets for proper attraction onto the receiving medium. Similarly, if there is an irregularity in either the shape or hardness of the doctor blade, there can be lateral nonuniformities in the ink level which can cause improper development. Non-uniformities of the fluid in the pockets of the gravure also alters the critical distance between the receiving medium and the development fluid. If this distance becomes too great, the electrostatic forces may be insufficient to draw the fluid to the recording medium. On the other hand, if the distance is too small, development fluid would be attracted to areas on the recording medium where fluid should not be deposited. Accordingly, it is a further object of the invention to provide an improved fluid development system wherein the critical distance between the receiving medium and the development fluid carried by the gravure roll is not changed upon doctoring the gravure.